Liposomes are known in the art to function as carriers to deliver therapeutic agents to targeted cells for treating a variety of medical conditions. In one application, liposomes may be formulated to encapsulate a pharmaceutical agent that can be phagocytized selectively by macrophages. Once phagocytized, the liposome releases the agent intracellularly, inhibiting inflammatory functions of the macrophages, among other effects.
The art describes several methods for preparing such liposomes (see, e.g., Mönkkönen, J. et al., 1994, J. Drug Target, 2:299-308; Mönkkönen, J. et al., 1993, Calcif. Tissue Int., 53:139-145; Lasic D D., Liposomes Technology Inc., Elsevier, 1993, 63-105. (chapter 3); Winterhalter M, Lasic D D, Chem Phys Lipids, 1993; 54(1-3):35-43). In one such method, liposomes are formed into stacks of liquid crystalline bilayers that are hydrated into hydrated lipid sheets, which detach during agitation and self-close to form large, multilamellar vesicles (MLV) known as thin lipid film hydration technique. Once these particles are formed, the size of the particle is dependent on the method used in the next steps of the process, for example, sonic energy (sonication) or mechanical energy (extrusion). Sonication typically produces small, unilamellar vesicles (SUV) and requires bath and/or probe tip sonicators. Alternatively, lipid extrusion which forces a lipid suspension through a series of polycarbonate filter—typically 0.8, 0.4, 0.2 and 0.1 μm membranes—at high pressure (up to 500 psi), produces particles having a diameter near the pore size of the filter used. These methods are limited to small-batch productions, primarily used for research purposes. Moreover, the high pressure extrusion techniques are associated with high operating costs and time. Thus, there remains a need in the art for a method of liposome production useful for commercial scale manufacture which is reproducible and addresses issues including quality control, stability, scalability and sterilization of liposome production.
In addition, liposomal formulations known in the art are not substantially uniform in size and shape which is a critical feature of a pharmaceutical composition to provide a sterile product and avoid potentially toxic side effects of aberrant large liposomes. Currently, it is difficult to manufacture a liposome formulation having a uniform size. The extrusion process of multilamellar vesicles through a series of filters including, for example, 100 nm polycarbonate filters does not consistently produce a formulation having substantially uniform population of liposomes having a 100 nm size. Indeed, depending on the physical characteristics of the liposomes, such as compressibility and/or stability, the mean vesicle diameter for extruded vesicles may vary considerably depending on the type and size of filters used. Thus, there is a need for a method of manufacture capable of producing liposomes substantially uniform in size and shape.
Moreover, many physical characteristics of the liposome formulation affect the cellular response to the liposome and impact the effectiveness of the liposome as a pharmaceutical composition. The physical characteristics of the liposome formulation are influenced in many respects by the manufacturing process. However, the art does not address how the liposomal properties can be controlled in the formulation process to manipulate manufacturing efficiency and liposome stability. Thus, there is a need for a large-scale manufacturing process that is cost effective, yet can control the characteristics of the liposome formulation to produce liposomes that are substantially uniform and suitable for clinical use.